I think I'd prefer this faction to not have a defensive unit at all. Ice age people wouldn't have had much time or the materials to build up fortifications since they were always on the move, following herds of migratory prey. In my mind, the troops should be pretty strong and tough because of the difficulty in surviving (and to compensate for their lack of defense), but their ranged weapons would be restricted to spears and atlatls. Also, as someone said previously, their buildings would be quick and cheap, but very flimsy (most would have wood or leather armor). One thing that I haven't seen anyone address yet is whether these people are supposed to be Cro-Magnon or Neanderthal. Neanderthals were of a sturdier build, so they would have more HP, but their spears weren't as sharp or accurate, so their ranged capacity would be less. It's also thought by many paleoanthropologists that Neanderthals may have died out because they couldn't breed as fast as Cro-magnon, so giving them a slow production speed would make sense. So which are they? I wouldn't suggest making them another hominid species, as this would limit your possibilities a lot more.